The invention relates to a device for detecting data relating to the passage of vehicles on a road, said device comprising a piezoelectric cable and means making the cable rigid and of transmitting loads produced by passing vehicles to said cable.
The invention is used for detecting the passage of vehicles, counting their numbers, measuring the speed and the weight of the vehicles and their classification in regions having different and, in particular, rigorous climatic conditions (a very hot, moderate or very cold climate).
In French Patent Specification FR 2 482 340 a description is given of a device for detecting the passage on a road. Further, it describes how it is installed and how it is used to measure speed.
Said device comprises, inter alia, a cable with a piezoelectric effect. This cable is a coaxial armoured cable the core and the cladding of which are separated by a piezoelectric ceramic.
Said device further comprises a rigid section having a U-shape or the shape of a rectangular quadrangle with a convex upper surface. Said section is filled on the inside with a synthetic resin material which is hardened by polymerization. The aim is to provide a rigid device.
In another version, the device is surrounded, at the three faces which are not in contact with the vehicles, with a U-shaped, flexible or hard synthetic resin envelope of, for example, a resin which is filled with an elastomer to absorb vibrations. This is a factory-made construction.
In either case, the installation of the device in the road surface requires the formation of a groove in the road, the dimensions of the groove being slightly larger than the dimensions of the device. Subsequently, the bottom and the sides of the groove are coated with a shock-absorbing, flexible synthetic resin material and, finally, the device is fixedly arranged in the coated groove while projecting slightly from the surface of the road. The coating of the groove can be dispensed with because the device is manufactured with a synthetic resin envelope.
To measure speed, two devices may be manufactured constituting two rigid, parallel limbs which are spaced at a predetermined, fixed distance from each other and arranged in two grooves in the road.
A disadvantage of the above-mentioned device according to the state of the art is that its manufacture requires the use of synthetic resin materials which are organic materials and which are very sensitive to variations in temperature.